1. Field of the Invention
The present invention relates to a magnetoresistive element and magnetic memory, e.g., a magnetoresistive element capable of recording information by supplying a current in two directions, and a magnetic memory using the same.
2. Description of the Related Art
The magnetoresistive effect is applied to a hard disk drive (HDD) as a magnetic memory device and presently put into practical use. The GMR (Giant Magnetoresistive) effect or TMR (Tunneling Magnetoresistive) effect is applied to a magnetic head of the HDD. Both the GMR effect and TMR effect detect a magnetic field from a magnetic medium by using a resistance change caused by an angle the magnetization directions in two magnetic layers make with each other.
Recently, various techniques have been proposed to implement a magnetic random access memory (MRAM) by using a GMR element or TMR element. An example is a method of recording information “1” or “0” in accordance with the magnetization configuration of an MTJ (Magnetic Tunnel Junction) element, and reading the information by using the resistance change caused by the TMR effect. Various techniques have been proposed to put an MRAM using this method into practical use as well. An example is a magnetic field writing method that switches the direction of magnetization in a magnetic layer by using a magnetic field generated by a current. A magnetic field generated by a current naturally increases as the current increases. However, due to advances in micropatterning, a current that can be supplied to an interconnection is limited. It is possible to increase the efficiency of a magnetic field generated by a current and reduce a current value necessary to switch magnetization in a magnetic layer by decreasing the distance between an interconnection and the magnetic layer or by using a yoke structure that concentrates the generated magnetic field. However, micropatterning increases a magnetic field required for magnetization reversal in a magnetic field. This makes it very difficult to reduce a current and advance micropatterning at the same time.
Micropatterning increases a magnetic field necessary for magnetization reversal in a magnetic layer because the magnetic energy that overcomes a thermal disturbance is necessary. This magnetic energy can be increased by increasing the magnetic anisotropic energy density and the volume of the magnetic layer. Since micropatterning reduces the volume, however, a general approach is to increase the magnetic shape anisotropic energy or magnetocrystalline anisotropic energy, i.e., increase the magnetic anisotropic energy density. Unfortunately, an increase in magnetic energy of a magnetic layer increases the reversing magnetic field as described above. This makes it very difficult to reduce a current and advance micropatterning at the same time.
Recently, magnetization reversal caused by a spin-polarized current has been theoretically predicted and experimentally confirmed, and an MRAM using a spin-polarized current has been proposed. This method can switch magnetization in a magnetic layer solely by supplying a spin-polarized current to the magnetic layer. If the volume of the magnetic layer is small, smaller spin-polarized electrons need be injected. Therefore, the method is expected to advance micropatterning and reduce a current at the same time. In addition, the method does not use any magnetic field generated by a current. Accordingly, the method has the advantage that the cell area can be reduced because no yoke structure for increasing a magnetic field is necessary. However, even in the magnetization reversing method using a spin-polarized current, the problem of thermal disturbance naturally arises as micropatterning advances.
As described previously, the magnetic anisotropic energy density must be increased in order to secure a high thermal disturbance resistance. An in-plane magnetization arrangement that has been principally studied so far generally uses the magnetic shape anisotropy. In this case, the magnetic anisotropy is secured by using the shape. This makes a switching current sensitive to the shape, and increases the variation in switching current as micropatterning advances, thus posing a problem. To increase the magnetic anisotropic energy density by using the magnetic shape anisotropy, it is possible to increase the aspect ratio of an MTJ element, increase the film thickness of a magnetic layer, or increase the saturation magnetization in the magnetic layer.
Increasing the aspect ratio of an MTJ element is unsuitable to increase the capacity because the cell area increases. Increasing the film thickness or saturation magnetization of a magnetic material is not preferable because the value of a spin-polarized current required for magnetization reversal increases. When using not the magnetic shape anisotropy but the magnetocrystalline anisotropy in the in-plane magnetization arrangement, if a material (e.g., a Co—Cr alloy material used in a hard disk medium) having a high magnetocrystalline anisotropic energy density is used, the crystallographic axis largely disperse in the plane. This decreases the MR (Magnetoresistive), or induces an incoherent precession. As a consequence, the switching current increases.
By contrast, when using the magnetocrystalline anisotropy in a perpendicular magnetization arrangement, it is possible to suppress the dispersion of the crystallographic axis that is the problem in the in-plane magnetization arrangement. For example, the crystal structure of the Co—Cr alloy material described above is the hexagonal structure, and has uniaxial magnetocrystalline anisotropy whose axis of easy magnetization is the c axis. Therefore, the crystal orientation need only be controlled such that the c axis is parallel to a direction perpendicular to the film surface. In the in-plane magnetization arrangement, the c axis must be uniaxially arranged in the film surface, and the rotation of each crystal grain in the film surface rotates the crystallographic axis and disperses the uniaxial direction. In the perpendicular magnetization arrangement, the c axis is perpendicular to the film surface. Accordingly, even when each crystal grain rotates in the film surface, the c axis remains perpendicular and does not disperse. Similarly, a perpendicular magnetization MTJ arrangement can be implemented by controlling the c axis in the perpendicular direction in the tetragonal structure as well. Examples of a magnetic material having the tetragonal structure are an Fe—Pt ordered alloy, Fe—Pd ordered alloy, Co—Pt ordered alloy, Fe—Co—Pt ordered alloy, Fe—Ni—Pt ordered alloy, and Fe—Ni—Pd ordered alloy each having the L10 crystal structure. When implementing a perpendicular magnetization MTJ arrangement by using the magnetocrystalline anisotropy, the aspect ratio of an MTJ element can be 1. Therefore, this MTJ arrangement is suitable for micropatterning as well.
A high magnetoresistive ratio is necessary to increase the capacity of an MRAM. Recently, many MTJ elements using MgO as a barrier material having a high magnetoresistive ratio have been reported, and it is regarded as important that the (100) plane of MgO orients in order to achieve a high magnetoresistive ratio. MgO has an NaCl crystal structure, and its (100) plane is favorable from the viewpoint of lattice matching with the (001) plane of the L10 structure. Accordingly, the use of an L10 perpendicular magnetization film as a magnetic layer in a perpendicular magnetization MTJ element is very promising from the viewpoint of the magnetoresistive ratio.
To use the L10 structure as a perpendicular magnetization film, however, the crystal orientation must be controlled such that the (001) plane orients. This requires an underlying layer for controlling the crystal orientation. In the magnetization reversing method using a spin-polarized current, the resistance of an MTJ element must be decreased because a current flows through a barrier layer. This makes it unfavorable to use an underlying layer having a high resistance. Also, to achieve a high magnetoresistive ratio, it is undesirable to use an element that decreases the magnetoresistive ratio as the material of the underlying layer, because the influence of diffusion occurring in a heating step necessary to order a magnetic layer into the L10 structure is significant.
As described above, if magnetization reversal by a spin-polarized current is achieved in a perpendicular magnetization MTJ element, it is possible to reduce a write current, secure a high thermal disturbance resistance of bit information, and reduce the cell area at the same time. In addition, a high magnetoresistive ratio can be obtained if it is possible to form an MTJ element using a magnetic material having the L10 structure that is favorable from the viewpoint of lattice matching with the MgO barrier. However, neither a report nor a practical method having achieved a high magnetoresistive ratio by forming an MTJ element by using a magnetic material having the L10 structure has been proposed.